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<b>请注意:</b><br>本书基于 Elasticsearch 2.x 版本，有些内容可能已经过时。
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<span class="breadcrumb-node">等待中的任务</span>
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<h2 class="title">
<a id="_pending_tasks"></a>等待中的任务<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/500_Cluster_Admin/40_other_stats.asciidoc">edit</a>
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<p>有一些任务只能由主节点去处理，比如创建一个新的索引或者在集群中移动分片。由于一个集群中只能有一个主节点，所以只有这一节点可以处理集群级别的元数据变动。在 99.9999% 的时间里，这不会有什么问题。元数据变动的队列基本上保持为零。</p>
<p>在一些 <code class="literal">罕见</code> 的集群里，元数据变动的次数比主节点能处理的还快。这会导致等待中的操作会累积成队列。</p>
<p><code class="literal">等待中的任务</code> API  会给你展示队列中（如果有的话）等待的集群级别的元数据变更操作：</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">GET _cluster/pending_tasks</pre>
</div>
<p>通常，响应都是像这样的：</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
   "tasks": []
}</pre>
</div>
<p>这意味着没有等待中的任务。如果你有一个罕见的集群在主节点出现瓶颈了，等待中的任务列表可能会像这样：</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
   "tasks": [
      {
         "insert_order": 101,
         "priority": "URGENT",
         "source": "create-index [foo_9], cause [api]",
         "time_in_queue_millis": 86,
         "time_in_queue": "86ms"
      },
      {
         "insert_order": 46,
         "priority": "HIGH",
         "source": "shard-started ([foo_2][1], node[tMTocMvQQgGCkj7QDHl3OA], [P],
         s[INITIALIZING]), reason [after recovery from gateway]",
         "time_in_queue_millis": 842,
         "time_in_queue": "842ms"
      },
      {
         "insert_order": 45,
         "priority": "HIGH",
         "source": "shard-started ([foo_2][0], node[tMTocMvQQgGCkj7QDHl3OA], [P],
         s[INITIALIZING]), reason [after recovery from gateway]",
         "time_in_queue_millis": 858,
         "time_in_queue": "858ms"
      }
  ]
}</pre>
</div>
<p>可以看到任务都被指派了优先级（ 比如说 <code class="literal">URGENT</code> 要比 <code class="literal">HIGH</code> 更早的处理 ），任务插入的次序、操作进入队列多久，以及打算处理什么。在上面的列表中，有一个 <code class="literal">创建索引(create-index)</code> 和两个 <code class="literal">启动分片(shard-started)</code> 的操作在等待。</p>
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<p class="title"><strong>什么时候应该担心等待中的任务？</strong></p>
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<p>就像曾经提到过的，主节点很少会成为集群的瓶颈。唯一可能成为瓶颈的是集群状态非常大 <em>而且</em> 更新频繁。</p>
<p>例如，如果你允许客户按照他们的意愿创建任意的动态字段，而且每个客户每天都有一个独立索引，那么你的集群状态会变得非常大。集群状态包括 ( 但不限于 ) 所有索引及其类型，以及每个索引的全部字段。</p>
<p>所以如果你有 100000 客户，然后每个客户平均有 1000 个字段，而且数据有 90 天的保留期—这就有九十亿个字段需要保存在集群状态中。不管它何时发生变更，所有的节点都需要被通知。</p>
<p>主节点必须处理这些变动，这需要不小的 CPU 开销，加上推送更新的集群状态到所有节点的网络开销。</p>
<p>这就是那些可以看到集群状态操作队列上涨的集群。没有简单的办法可以解决这个问题，不过你有三个选择：</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
使用一个更强大的主节点。不幸的是，这种垂直扩展只是延迟这种必然结果出现而已。
</li>
<li class="listitem">
通过某些方式限定文档的动态性质来限制集群状态的大小。
</li>
<li class="listitem">
到达某个阈值后组建另外一个集群。
</li>
</ul>
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